In general, the present invention relates to an information outputting method, an information outputting apparatus adopting the method and a presentation medium for presenting the method. More particularly, the present invention relates to an information outputting apparatus capable of playing back a plurality of streams continuously with a high degree of reliability, an information outputting method adopted by the apparatus and a presentation medium for presenting the method.
In a digital television broadcast system such as a DVB (Digital Video Broadcasting) or a DTV (Digital Television), a transport stream of the MPEG2 systems (ISO/IEC 13818-1) is used in a multiplexing system. To be more specific, video and audio streams are multiplexed in a transport stream in transport-packet units each having a length of 188 bytes.
When transport packets of one or more television programs are extracted from a transport stream in which a plurality of television programs are multiplexed, the extracted transport packets appear at irregular intervals.
To put it in detail, in a stream transmitted through a satellite, transport packets of a plurality of channels are multiplexed as shown in FIG. 1. In the example shown in the figure, transport packets of 4 channels, namely, channels A to D, are multiplexed. Assuming that transport packets of channel A are extracted from the stream, timings with which transport packets A1 to A5 are generated are irregular. That is to say, an interval between two adjacent transport packets Ai and Ai+1 varies from time to time.
When such transport packets Ai are recorded into a recording medium at the irregular intervals as they were extracted, however, the storage area of the recording medium is not utilized efficiently. Thus, transport packets Ai need to be recorded by closing gaps between them.
If transport packets Ai are thus recorded by closing gaps between them, however, it is necessary to adjust timings, with which transport packets Ai are output in an operation to play back the transport packets Ai from the recording medium, to the timings to which the packets have been transmitted. Otherwise, timing to supply the packets to an T-STD (Transport-System Target Decoder) prescribed by the MPEG2 standards will be different from the timing to encode the packets. As a result, it will be quite within the bounds of possibility that the T-STD fails. A conceivable solution to this problem is to also record a time stamp Tsi representing timing of generation of each transport packet in the transport packet at the same time as shown in FIG. 1.
FIG. 2 is a block diagram showing a typical configuration of an apparatus for recording a transport stream by embracing such a principle. In a bitstream parser 2, a program clock reference (PCR) is read out from the header of a transport packet of a transport stream received through a terminal 1. The transport packet is supplied to a time-stamp adding circuit 3 while the PCR is supplied to a PLL (phase-locked loop) circuit 4. The PLL circuit 4 generates a clock signal with a frequency of 27MHz in synchronization with the PCR supplied thereto, outputting the clock signal to a timestamp generating circuit 5. The time-stamp generating circuit 5 counts the number of pulses of the clock signal and generates a time stamp (time_stamp_counter) corresponding to a count value to the time-stamp adding circuit 3 to output.
The time-stamp adding circuit 3 adds 3-byte TSP_extra_header to 187 bytes of the transport packet to generate a 190-byte block as shown in FIG. 3. The 187 bytes are obtained by removing a sync byte at the beginning of the original 188-byte transport packet. As shown in FIG. 4, TSP_extra_header includes 21-byte time_stamp_counter. time_stamp_counter represents a planned arrival time of the first bit of the first byte of the transport packet at a smoothing buffer defined in the ISO/IEC13818-1.
It should be noted that the format shown in FIG. 3 is a format for a transport-stream recording apparatus functioning as a digital VCR (Video Cassette Recorder) for use of consumer.
Data output by the time-stamp adding circuit 3 is supplied to a storage media unit 7 by way of a smoothing buffer 6 to be recorded therein.
A transport stream recorded in the storage media unit 7 in this way is played back by a playback apparatus having a configuration shown in FIG. 5. As shown in the figure, a transport stream played back from the storage media unit 7 is supplied to a time-stamp separating circuit 21 for separating a transport packet and time_stamp_counter from the transport stream. The transport packet is supplied to an output control circuit 23 whereas time_stamp_counter is supplied to a comparator 22 and a timing generating circuit 24.
In the timing generating circuit 24, the number of clock pulses generated by a clock generating circuit 25 at a frequency of 27 MHz is counted by a counter with the value of first time_stamp_counter used as an initial value. A count value representing a time stamp is supplied to the comparator 22. When the count value received from the timing generating circuit 24 becomes equal to the value of time_stamp_counter received from the time-stamp separating circuit 21, the comparator 22 controls the output control circuit 23 to output the transport packet received from the time-stamp separating circuit 21 to a terminal 26.
To be more specific, when the count value received from the timing generating circuit 24 is found equal to TS1 by the comparator 22, transport packet A1 is output from the output control circuit 23. By the same token, when the count value received from the timing generating circuit 24 becomes equal to TS2, transport packet A2 is output from the output control circuit 23. Thereafter, the same processing is repeated. Thus, transport packets Ai are each output with the same timing as the encoding operation. As a result, it is possible to prevent the TSTD from failing.
When a stream of channel A and a stream of channel H are synthesized in editing work as shown in FIG. 6, however, continuity at a junction between the two streams is lost due to the fact that there is no relation between a time stamp TSn of the last transport packet An of channel A and a time stamp TS11 of the first transport stream packet H1 of channel H.
As a result, when data obtained as a result of editing a plurality of streams in this way and recorded on storage media is played back, at the junction between the last transport packet An of channel A and the first transport packet H1 of channel H, a time stamp output by the timing generating circuit 24 does not correspond to a time stamp output by the time-stamp separating circuit 21, raising a problem that it is no longer possible to correctly control the output control circuit 23.